Many of the features in the ontogeny of behavior are common to all animals with a central nervous system. For instance, spontaneous coordinated movements appear before reflexes can be elicited in many animals, from insects and annelids through frogs, chicks and humans. The spontaneity has been attributed to the establishment of interneuronal connections to motor neurons before the sensory to interneuronal connections form. By using a relatively simple animal, the leech, in which both the structure and function of individual neurons has been well characterized, and which has a nervous system that is accessible for cellular studies throughout development, we can determine the cellular basis for such behavioral observations. We propose to study the neuronal basis for the development of two leech behaviors, reflex shortening and swimming. Using intracellular microelectrodes, we will record from identified sensory neurons, interneurons and motor neurons when these behavioral acts appear and are elaborated. We will characterize their individual electrical properties, particularly their ability to generate action potentials. We will also determine when they first make synaptic contacts, and follow the progress of the synaptic properties until they acquire all their adult-like features. In addition, we will fill the neurons with dyes, to characterize at both the light and electron microscopic levels the outgrowth of the neuronal processes and the ways in which processes of different cells come together to make synapses. Once we know the normal order of developmental events, we will perturb that order by cutting nerves, ablating target tissues, and by killing individual neurons to establish the rules by which neurons connect to one another to form circuits responsible for producing behavior. Hopefully, these studies will provide insight into the behavioral development of other animals, and will also provide a basis for studying the molecular mechanisms for the formation and maintenance of synapses in the development of behavior.